The invention relates to an indicator for proper or improper exposure by an automatic electronic flash, and more particularly, to such indicator which indicates the result of an actual exposure provided by an automatic electronic flash, giving an indication of an overexposure, underexposure or proper exposure.
As is well recognized, when taking a picture with the aid of an automatic electronic flash, the amount of light emission from the electronic flash can be controlled within a limited range. The control of the electronic flash tends to result in an overexposure as a distance to an object being photographed decreases or as a diaphragm aperture increases toward an open value. For example, in an automatic electronic flash of series controlled type, if an emission terminate signal is applied in synchronism with a flash emit signal, there results a finite or minimum emission of light from the electronic flash rather than resulting in zero emission. This is attributable to the nature of a forced commutation circuit including a thyristor of an automatic emission control circuit contained in the electronic flash. If the emission terminate signal causes the thyristor to conduct, it takes a given magnitude of turn-on time which is inherent to the thyristor. Recently, TTL automatic electronic flash is in practical use which is controlled in accordance with an output from an automatic exposure control circuit of a camera of TTL (through-the-lens) direct photometry type. This permits the implementation of a variety of lighting set-ups or the use of multiple electronic flashes in an automatic emission control mode, by interconnecting the camera and the electronic flash or flashes by means of electric cords. However, a transmission lag due to transmission lines running between the camera and the emission control circuit of the electronic flashes or the multiple lighting causes an increase in the minimum emission. For the sake of reference, representing the minimum emission of each individual electronic flash by G.sub.NO L, the minimum emission where N electronic flashes are used to provide a multiple lighting will be equal to G.sub.NO L.times..sqroot.N. When the minimum emission increases in this manner, an amount of exposure Ev given by actual light emission from electronic flash or flashes will be as indicated by a curve L in FIG. 1. Thus, the amount of exposure will deviate from a proper exposure level L.sub.0 toward the overexposure as the distance to an object being photographed decreases. The overexposure renders it impossible to take a picture with the aid of an electronic flash in a range of distance to an object being photographed which is located nearer than a point P toward the photographer. On the other hand, the maximum emission of the electronic flash will be exceeded with a distance to an object being photographed which is located further beyond a point D, resulting in an underexposure.
A variety of indicators for providing an indication of proper or improper exposure given by an automatic electronic flash are known, including
1. an apparatus for providing an indication of proper or improper exposure based upon the comparison against a first level corresponding to proper exposure and a second level which represents a given amount of overexposure, of an output from an integrator which integrates the amount of light incident (see Japanese Utility Model Publication No. 2,977/1982);
2. an apparatus for providing an indication of proper or improper exposure based upon a determination if an output from the integrator has reached a proper level at time which occurs a given time interval after the initiation of the emission of flashlight (see Japanese Laid-Open Patent Application No. 156,620/1977);
3. an apparatus including means to give a warning of underexposure at a given time interval after the initiation of emission of the flashlight (or closure of X-contacts), and in which the warning means is disabled if the electronic flash has ceased to emit flashlight at a proper exposure level (see Japanese Utility Model Publication No. 19,075/1981).
With the apparatus mentioned above sub-paragraph 1 above, the amount of incident light supplied from the electronic flash is monitored to determine and to indicate if an overexposure level, which is a given value above a proper exposure level, is reached. With this apparatus, while a given latitude from the proper level is considered for the overexposure, no latitude is considered in the determination of the underexposure. Consequently, if the integral of an amount of light incident when the electronic flash is allowed to provide a full emission is less than the proper level by an amount which is as little as 0.1 Ev, the apparatus provides an indication of underexposure. Accordingly, there results an unbalanced criterion of the overexposure and the underexposure. In practice, it will be desirable that a given latitude should be allowed for the underexposure in the same manner as such latitude is allowed for the overexposure. For practical purposes, a satisfactory photographing operation is achieved with a latitude on the order of -1/2 Ev to -1/3 Ev. It should also be noted that the integration of an amount of incident light during a photographing operation with the aid of an electronic flash is normally initiated in synchronism with the initiation of light emission from the electronic flash, and thus a component of an integral which is attributable to steady-state light, though of relatively low brightness, which exists until the first blind of the shutter becomes fully open, is discarded from the determination. To take this into consideration, it is reasonable to allow a given latitude for the underexposure.